In this type of technical field, a network scheme called an ad hoc network, a multi-hop network, a mesh network or the like is receiving attention. In the ad hoc network, neighboring wireless nodes can directly communicate with each other without a fixed infrastructure such as access points (APs) or wireless base stations, and it is allowed to pass through one or more wireless nodes for transmitting information to a wireless node of a communication partner.
According to the ad hoc network, a wireless network can be flexibly constructed even in a place where the fixed infrastructure is not provided. Therefore, it holds promise as an economical network constructing method. The ad hoc network can be utilized not only as an indoor communication environment such as a home or an office but also as an outside private or public network such as a hot spot. In addition, since the ad hoc network does not require the fixed infrastructure, it is also effective for constructing a temporary network in a time of disaster.
FIG. 1 is a schematic diagram showing an overall view of an ad hoc network, and shows a plurality of nodes that are nodes A-G. The ad hoc network does not require an apparatus for centrally controlling or managing a plurality of nodes, but it is necessary that frequencies used for nodes related to the communication are the same. That is, a sending apparatus and a receiving apparatus in each node performing communication needs to operate with a same frequency, and each node needs to have a frequency synchronization function for selecting such a frequency.
For example, in a wireless LAN system described in a non-patent document 1, two communication modes are defined. One of them is called a BSS mode in which a communication terminal (STA: station) performs communication via a wireless base station that is an access point AP. In the BSS mode, the access point AP comprehensively determines a frequency used for communication, so that each communication terminal STA receives announcement information (also to be referred to as a beacon) that is sent periodically or irregularly to tune to the frequency used in the access point AP.
Another is called an iBSS mode in which communication terminals STA are directly connected with each other by a wireless line to perform communication under a condition that all communication terminals STA are placed within a range where a radio wave of each communication terminal reaches each other communication terminal. In the iBSS mode, a node checks whether it can receive a beacon sent by another node, and when the node cannot detect the beacon sent by the another node, the communication terminal STA (the node) determines a frequency to be used to send a beacon. In addition, when the node can detect a beacon sent from another node, the communication terminal tunes itself to the frequency indicated by the beacon.
In the Bluetooth indicated in a non-patent document 2, there are two categories that are a master and a slave for a node, and a node functioning as the master determines a frequency so that a node of the slave synchronizes the frequency according to the frequency determined by the master. In this case, only one master can exist in a network (pico-network).                [Non Patent document 1] The Institute of Electrical and Electronics Engineers, Inc., (“IEEE”), “IEEE Wireless LAN Edition—A compilation based on IEEE Std 802.11TM-1999 (R2003) and its amendments—”, 2003        [Non Patent document 2] J. Bray, Charles F Sturman, “Bluetooth Connect Without Cables”, Prentice-Hall, Inc, 2001        